1. Field of the Invention
The present invention relates to a semiconductor abrupt junction having a thin recombination layer immediately adjacent to the effective junction. More particularly, the invention relates to a semiconductor junction having a shallow, highly doped region of one conductivity type, a relatively lightly doped region of opposite conductivity type and, between the two regions, a thin recombination layer of first conductivity type and of dopant concentration intermediate that of the heavily and lightly doped regions.
2. Description of the Prior Art
The operational speed of computers and other electronic circuitry rapidly is approaching the limitations inherent in individual circuit components. Thus, while the circuit designer is under ever increasing pressure to develop faster and faster circuitry, components are not available which will facilitate such high-speed operation. Significant among the items which limit circuit speed is the p-n junction, the basic constituent of semiconductor devices such as diodes, transistors and the like.
The period of time required to bring a prior art semiconductor junction into full forward conduction typically may range from several nanoseconds to a few microseconds. This forward recovery time (T.sub.fr) may correspond to a substantial portion of a circuit operational duty cycle. Moreover, the reverse recovery time (T.sub.rr), which is related to the time required to turn off a semiconductor diode, typically may range from about ten nanoseconds to about one hundred microseconds for prior art p-n junctions.
The forward and reverse recovery times of a p-n junction are important not only when a device is used in a switching or other computer circuit, but also when used for clamping. In such an application, if a diode is subjected to a voltage transient faster than the recovery time of the device, the transient will not be clamped. Of course, this defeats the purpose of utilizing the clamping diode, and may cause erroneous circuit operation or damage to other circuit components exposed to the unclamped transient voltage.
The ability of a clamping diode to react to very high-speed transients is particularly important in certain special purpose circuits such as the flyback dc-dc regulator. This circuit is very useful for controlling or regulating power, but requires diodes which exhibit very fast turn-on and turn-off times, and which also are capable of handling high forward current. Such diodes have not been available in the past.
In addition to fast forward and reverse recovery times, there are cognate characteristics desirable for semiconductor p-n junctions. For example, it is advantageous for such junctions to exhibit very low forward turn-on voltage. While a forward turn-on voltage of 0.2 volts at one milliampere may be exhibited by germanium diodes at 25.degree. C., at the same current and temperature, prior art silicon diodes typically exhibit a turn-on voltage of 0.6 volts. Other desirable characteristics are that the junction should be capable of handling high current during steady state forward conduction, should exhibit low reverse current while maximizing the forward characteristics described above, and should have high stability in a radiation environment.
As discussed in detail below, these characteristics cannot simultaneously be obtained in prior art semiconductor p-n junctions. For example, consider the transient phenomenon associated with switching a prior art junction diode from forward conduction to reverse bias conditions. While the junction is conducting a steady current in the forward direction, excess minority carriers are present in the n and p regions of the junction. The minority carrier density is at a maximum adjacent the boundary with the forward biased depletion region, and decreases with increasing distance from that boundary.
When the bias voltage across the diode is reversed, the excess minority carriers first must be cleared from the n and p regions, by diffusion into and recombination within the depletion region, before the steady state reverse current condition is reached. During this reverse recovery time (T.sub.rr), excess current flows in the reverse direction. The time required to clear the junction of stored excess minority carriers depends on the junction geometry, the dopant concentrations in the p and n regions, the perfection of the semiconductor matrix, and the concentration of recombination sites in the structure.
One approach widely used in the prior art to speed up the reverse recovery time of a p-n junction diode is to introduce throughout the diode an impurity dopant having an energy level near the middle of the semiconductor forbidden gap. Such impurities, examples of which are copper in germanium and gold in silicon, therefore act as efficient recombination-generation centers which speed up the clearing of excess minority carriers upon junction turn-off, and hence act to reduce the junction reverse recovery time. However, the presence of such gold or copper foreign material significantly reduces the lifetime of holes or electrons within the depletion region of the junction when reversed biased, and thereby significantly increases the steady state reverse current of the junction. Thus, this prior art technique improves one junction characteristic (reverse recovery time) at the expense of degrading another characteristic (reverse leakage).
No satisfactory technique has been available in the past to improve the forward and reverse switching characteristics of a semiconductor p-n junction without impairing other performance characteristics of the junction. In contradistinction, the present invention comprises a semiconductor junction and technique for making same; the inventive junction exhibits forward and reverse switching characteristics significantly faster than previously available. At the same time, the novel junctions provide improved forward turn-on characteristics, high steady state forward current handling ability and inherently high stability in radiation environments, all while maintaining good reverse voltage and current characteristics.